Black shut off electric switch in Broadway EB

Given that your 2016 Broadway has a solar panel, that should be sufficient at this time of year to keep both batteries adequately charged (especially if the solar panel is the larger capacity one). You need do nothing. Do not select battery on the Sargent control panel (i.e. do not have the tell-tale blue light on): the current from the panel will be shared between both batteries, keeping them OK.

However, if it's possible to connect to mains, then as Mids says, a regular charge from the mains will be adequate.

I'm afraid I don't know what you're referring to as the "black electric shut down switch for the van." Is it on the Sergant box? A precise description, or better a photograph, would greatly assist. But be that as it may, I think you'll probably be OK by leaving well alone - as long as you've nothing running on the 12v system (such as an alarm or lights).

Cymro

PS: Having started to type above, I nipped out to my van to search for a black switch. Now I'm back and have sent this, I've seen Peter's. I have the 328, not the 500 system, so I have no black switch!  Hence this postscript.